Anti-holdover charging circuit for flash lamp

ABSTRACT

Charging circuit for a flash lamp in which flash holdover is prevented by establishing a high impedance charging circuit for a predetermined time after flash initiation to allow deionization of the lamp plasma and, after a predetermined time, establishing a low impedance charging circuit to provide a fast charging rate for the energy storage device. Impedance levels are changed in the circuit by gating a silicon controlled rectifier (SCR) with a control signal a predetermined time after flash initiation so as to bypass a high impedance circuit path in parallel therewith. As the energy storage device becomes fully charged, the SCR returns to its blocking condition, the control signal terminates and the lamp can be fired again.

United States Patent [191 Deskevich et al.

[ ANTI-HOLDOVER CHARGING CIRCUIT FOR FLASH LAMP [75] Inventors: Stephen Deskevich, Endwell; James C. Peacock, Endicott; Richard F. Tynan, Endwell; Alan D. Wilson, Apalachin, all of NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: June 26, 1973 [21] Appl. NO.: 373,848

[451 May 6,1975

3,777,212 12/1973 Mashimo ..3l5/24IP Primary Examiner-R. V. Rolinec Assistant Examiner-Lawrence J. Dahl Attorney, Agent, or FirmK. P. Johnson [57] ABSTRACT Charging circuit for a flash lamp in which flash holdover is prevented by establishing a high impedance charging circuit for a predetermined time after flash initiation to allow deionization of the lamp plasma and, after a predetermined time, establishing a low im- [52] U S Cl 315/241 315/240 320/1 pedance charging circuit to provide a fast charging [51] H05; 37/00 rate for the energy storage device. Impedance levels Field p 241 e Changed in the circuit by gating a silicon con- 320/1 trolled rectifier (SCR) with a control signal a predetermined time after flash initiation so as to bypass a [56] References Cited high impedance circuit path in parallel therewith. As the energy storage device becomes fully charged, the UNITED STATES PATENTS SCR returns to its blocking condition, the control sig- :iieder na] terminates and the lamp can be fired again 5,4 8 ieder l5 2 3,764,849 9/1973 Ohta 315/241 P 4 Claims, 3 Drawing Figures u 10 24 1 ab CURRENT DRIVER E RATENTEU 65975 3,882,358

W 51 so FIGJ 17 i8 ,54 5k CURRENT DRIVER DELAY (a )FLASH TRIGGER TL (b )LAMP CURRENT c )PRIOR ART CHARGE CURRENT (d) PRIOR ART CHARGE VOLTAGE 5T (e)CHARGE CURRERTM (f)CHARGE VOLTAGE W (9 )CURRENT DRIVER 3W FIG. 2

. N v n 54 55 2a 29 CURRENT DRIVER DELAY FIG. 3

ANTI-HOLDOVER CHARGING CIRCUIT FOR FLASH LAMP BACKGROUND OF THE INVENTION Known charging circuits for flash lamps at times ex hibit flash holdover or prolongation of the flash period. Holdover is due to the action of the charging circuit which begins to supply charging current immediately upon initiation of the flash. When the current supply is sufficient. discharge of the lamp will continue and the storage capacitor will not charge. Because of this action. the repetition rate of the flash lamp is restricted.

The solution to the holdover problem in the past has been a compromise of increasing the impedance of the charging circuit so as to limit the current available for flash discharge to produce extinction. thus preventing continued lamp conduction. However. this approach also increases the time constant of the charging circuit and decreases the maximum repetition rate for lamp discharge.

It is accordingly a primary object of this invention to provide a charging circuit for a flash lamp in which the charging circuit has selectable charging rates to control flash holdover during lamp discharge.

It is another important object of this invention to provide a novel charging circuit for a flash lamp in which charging current is severly limited during the discharge of the flash lamp and until the lamp is extinguished. and thereafter establishing a low impedance charging circuit to permit rapid recharging of the energy storage device.

Yet another important object of this invention is to provide a charging circuit for a flash lamp which has variable charging rates and which is simple. reliable and inexpensive.

SUMMARY OF THE INVENTION The foregoing objects are attained in accordance with the invention by providing in series with the conventional resistor-capacitor charging and storage circuit for a flash lamp. a high impedance path and a selectively switchable low impedance path in parallel with each other. A conventional triggering circuit is employed for discharge of the flash lamp. During lamp conduction, energy is drained from the storage capacitor which is attempted to be replaced by the power supply and. thus. sustain discharge through the ionized gas of the flash lamp. However. with the low impedance path maintained open. the high impedance charging path severely limits charging current for the storage capacitor below the level required for holdover. At a predetermined time after initiation of flashing. when discharge has terminated due to deionization. the low impedance gate control device is switched on so that charging can occur to replenish the energy storage capacitor. After charging has been completed. the low impedance path is again switched offin preparation for the subsequent flash initiating signal. Means are provided between the initiating signal and switchable low impedance gate to maintain a fixed time relation to insure that charging will not occur for the lamp circuit until lamp conduction has terminated.

The flash lamp circuit can be operated at either polarity merely by appropriately poling the few polarity sensitive devices for proper current flow. The use of the gate-controlled impedance device enables attainment of higher repetition frequencies for the flash lamp by permitting the use of a high impedance device in parallel with the controlled impedance. This. in effect. terminates current flow to the energy storage capacitor or flash lamp until dissipation of the previously stored charge. In addition. the circuit has the advantage of permitting the controlled impedance in the charging circuit to be optionally connected to the initiating trigger pulse or to some extraneous signal known to occur at a predetermined time after the trigger pulse. Also. with the controllable charging impedance. the usual R-C charging circuit can have a much shorter time constant to thus achieve a higher repetition rate.

The foregoing and other objects. features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a flash lamp circuit constructed according to the principles of the invention.

FIG. 2 is a diagram of electrical waveforms illustrating timing relations among various components in the circuit of FIG. 1 and prior art circuits.

FIG. 3 is a schematic diagram of an alternative embodiment of the circuit shown in FIG. 1.

Referring to FIG. 1. a flash lamp 10. with anode l1 and cathode 12. is connected in parallel with an energy storage capacitor I3 across a source 14 of potential. indicated as a battery. Connected between the positive terminal of the battery and the capacitor is resistor 15 which forms with capacitor I3 and R-C charging circuit for lamp 10. A diode 16 is placed between anode 11 and the junction of resistor I5 and the capacitor 13 to block reverse current flow during triggering of the lamp. as will be evident later.

Lamp cathode 12 is directly connected to the opposite side of storage capacitor 13 and to the anode of a silicon controlled rectifier (SCR) 17. The cathode is connected to the negative terminal of battery 14. The SCR has a control electrode 18 which can be pulsed for gating the SCR into conduction at selected times. The SCR also has a resistor 19 connected in parallel therewith. This resistor is preferably many times the impedance of resistor 15.

The flashing of lamp 10 is initiated through the amplification of a high voltage pulse supplied through a trigger circuit indicated generally as 20 to the right of lamp 10 in FIG. 1. Terminal 21 of the trigger circuit is connected to a source 32 of positive potential such as 100 volts and to current limiting resistor 22 which. in turn. is connected to a blocking capacitor 23 and the anode of an SCR 24 which has a control electrode 25 connected to an imput signal terminal 26. Capacitor 23 is connected in series with one terminal of the primary winding ofa transformer 27. The other terminal of the primary winding is connected in common at line 28 with the cathode of SCR 24 and thr trigger power source 32 at terminal 29. The secondary winding of transformer 27 is connected at one end to lamp cathode 12 and at the other end through a resistor 30 and capacitor 31 to anode ll of flash lamp 10. Input control terminal 26. in addition to being connected to control electrode 25 of SCR 24 also serves as an input to single shot circuit 33 which produces a narrow output pulse upon each activation which is supplied to a current driver 34. The current driver provides an output signal of predetermined duration which is supplied to control electrode 18 of SCR 17 to thereby switch the SCR to a conductive state for the duration of the driver output.

In operation. an input flash control signal is applied at terminal 26 and electrode which gates SCR 24 into conduction allowing discharge of capacitor 23 thus producing a pulse in the primary winding of transformer 27. A triggering pulse is thereby induced in the secondary winding and appears across resistor and across capacitor 31 as a high voltage spike superimposed on the potential existing at anode ll. Diode 16 isolates the pulse from the charging circuit. It may be assumed storage capacitor 13 is fully charged from source 14 in the R-C network of resistor 15 and capacitor 13. When the high voltage spike appears at anode 11, it is sufficient to produce ionization of the gas in lamp 10 and initiate discharge of capacitor 13 through diode 16 and flash lamp 10 back to capacitor 13. Lamp current resulting from the trigger pulse is shown in waveforms a and b of HO. 2. The dotted line in waveform b indicates the current level necessary to sustain conduction in flash tube 10.

Assume for the time being that SCR 17 and resistor 19 have been omitted from the charging circuit. ln this case, waveforms c and d. respectively. illustrate the current'flow and charging voltage during the recharging of storage capacitor 13. It will be seen that recharging occurs as soon as conduction is initiated within flash tube 10, and that potential source 14 will provide sufficient current to. sustain lamp conduction beyond the period desired resulting in'flash holdover.

Now assume that SCR l7 and resistor 19 have been placed as shown in FIG. 1. When the trigger control signal occurs at terminal 26 to turn on SCR 24, it also starts single shot circuit 33. The single shot produces an output signal for a predetermined time after being turned on. This time period is selected to be of sufficient length to allow the flash tube to deionize and. thus, stop conduction. The end of the delay produced by the single shot output turns on current driver circuit 34 which turns on SCR 17 at gate electrode 18. The effect of the delay in turning on SCR 17 is shown by waveforms e and fof FIG. 2, respectively, showing current flow and voltage change at storage capacitor 13.

While SCR 17 is non-conducting, the recharging path for the R-C network l5. 13, includes resistor 19 in parallel with the SCR. This resistor severely limits current flow in the recharging circuit. Thus. there is insufficient current to sustain flash holdover. When SCR 17 is later turned on by current driver 34 (waveform g), it provides a low impedance bypass of resistor 19. allowing a near-normal charging rate. Charging current flow at the time of SCR turn-on can be seen at point 36 on waveform e. As capacitor 13 reaches full charge. current flow diminishes to a point which will not sustain SCR 17 in a conducting condition as indicated by point 37. Thereafter, any remaining charging will occur through resistor 19 in parallel with the SCR. This maintains the capacitor fully charged during periods of infrequent flashing. Current driver 34 is preferably set to terminate its gating signal soon thereafter to thus insure that the SCR will be off before the next trigger pulse to initiate flashing of lamp 10.

The parallel current paths through resistor 19 and SCR 17 permit close control of the charging current to thereby insure that lamp 10 will not'exhibit holdover.

The amount of delay after the occurrence of an input flash control signal at terminal 26 depends upon the characteristics of the particular flash lamp being used. Such delay is usually in the order of a few milliseconds, at which time the lamp has deionized and charging can occur for storage capacitor 13. Although the gating signal for SCR 17 has been shown as coupled with the input flash initiation control signal. it can alternatively be supplied from various other sources which bear a known time relationship to the signals supplied at terminal 26. Such other signal sources may be a clock pulse generator or counter output, for example.

The circuit shown in FIG. 1 can be connected with the negative terminal of source 14 at ground potential. In this instance, the flash lamp cathode and capacitor terminals would be floated above ground when the SCR 17 turns off. They are substantially at ground when the SCR is on and the capacitor is charging. An alternative to this construction is shown in H0. 3. In this figure, cirucit elements idential with those in H6. 1 are given identical reference numerals. In the alternative embodiment. the cathode of SCR 17 and the flash lamp can be connected to ground. The positive terminal of source 14 will then be substantially at ground when SCR 17 is conducting. When the SCR is not conducting, the positive terminal of the source is floating. In this case, the lamp is inserted in the circuit reversed from FIG. 1. One terminal of the storage capacitor and lamp would always be at ground potential as in theconventional case when no SCR is present in the circuit. The alternative embodiment may be preferred because the lamp is not floated above ground during part of the flash cycle.

An advantage of the circuits disclosed in FIGS. 1 and 3 is that the impedance of resistor 15 can be decreased with the corresponding decrease in the charging circuit time constant for a given storage capacitor 13 so that the flash lamp may be operated at high flash frequency at the same energy pulse. This can occur without danger of flash lamp holdover since the charging voltage does not appear across the lamp until such time as the lamp is able to withstand that voltage. A voltage comparable to the holdover condition across the lamp but limited by a large resistor could be used to control the SCR gate current and insure that the lamp is in a nonconductive state when the SCR is turned on.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A circuit for permitting high frequency repetition of a flash lamp comprising:

an energy storage circuit connected to said lamp;

direct current charging means connected to said storage circuit. said charging means including a first constant impedance path and a second path with switch means having an on-state and an off-state in parallel with said first path to vary the rate of energy supplied to said storage circuit. said second path switch states being selectable in response to a control signal;

trigger pulse means for initiating flashing of said lamp and discharge of said storage circuit; and

6 quired to sustain flash holdover in said lamp.

3. Apparatus as described in claim 1 further including means for terminating said control signal a predetermined time after said switch means has been selected to said on-state.

4. Apparatus as described in claim 1 wherein said switch means is a gate-controlled semiconductor device. 

1. A circuit for permitting high frequency repetition of a flash lamp comprising: an energy storage circuit connected to said lamp; direct current charging means connected to said storage circuit, said charging means including a first constant impedance path and a second path with switch means having an on-state and an off-state in parallel with said first path to vary the rate of energy supplied to said storage circuit, said second path switch states being selectable in response to a control signal; trigger pulse means for initiating flashing of said lamp and discharge of said storage circuit; and circuit means including a delay circuit therein, said circuit means being connected to said trigger pulse means and responsive thereto for producing a said control signal at said charging means a predetermined time after said trigger pulse for selecting one of said switch states of said second path and changing said energy supply rate.
 2. Apparatus as described in claim 1 wherein the impedance of said first path is of a value limiting energy flow to said storage means at a rate less than that required to sustain flash holdover in said lamp.
 3. Apparatus as described in claim 1 further including means for terminating said control signal a predetermined time after said switch means has been selected to said on-state.
 4. Apparatus as described in claim 1 wherein said switch means is a gate-controlled semiconductor device. 